An optical transmitter may employ digital modulation to modulate transmitter laser light (i.e., an optical signal), to produce modulated laser light. In an example, digital modulation in the form of dual-polarization binary phase shift keying (DP-BPSK) modulates the laser light only on one axis, compared with other types of digital modulation, such as quadrature phase shift keying (QPSK) and 16 quadrature amplitude modulation (16QAM), which modulate the transmitter laser light onto orthogonal axes. An optical receiver demodulates the modulated laser light based on receiver laser light. Ideally, there is a non-zero frequency offset between the transmitter laser light and the receiver laser light, which improves demodulation. In practice, however, there may be zero or near-zero frequency offsets between the transmitter and receiver laser light. This occurs when the optical transmitter and receiver use different lasers that happen to generate laser light at the same frequency by chance or when the optical transmitter and receiver share a common laser.
To demodulate the modulated optical laser light, the optical receiver detects in-phase (I) and quadrature (Q) signals conveyed by the modulated optical laser light based on the receiver laser light. The above mentioned zero or near-zero frequency offsets cause energy fading in the I and Q signals, which hampers the demodulation process. For example, the fading perturbs automatic gain control loops used to control photo-detectors that detect the I and Q signals. The fading also complicates polarization tracking in cases where a constant modulus equalization algorithm is used. The fading is exacerbated by DP-BPSK modulation, which already limits energy to only one axis (either I or Q).